I wpuld like to suggest thos cactus trees might be some form of Prickly Pear?

That's what I was thinking, but all the prickly pear i have ever seen was much closer to the ground. I do know that there was a big prickly pear problem up in that region at the time that it was settled. My grandfather was granted an allotment of land at Wandoan in the early 1900s which apparently was to be granted free on the condition that the prickly pear was all removed, but he appears to have forfeited that land and instead enlisted in the AIF and went to Europe to join the WW1 fun.